2007 Annual Science Report

Carnegie Institution of Washington
Reporting | JUL 2006 – JUN 2007

Project 6. Molecular and Isotopic Biosignatures

Project Summary

Differences in carbon isotope ratios of co-occurring dolomite, carbonate fluorapatite, and organic matter in Paleoproterozoic stromatolitic phosphorites from the Aravalli Supergroup, India, suggest elevated primary productivity during sedimentation. Postdoctoral Fellow Dominic Papineau’s research provides a basis for the examination of other Paleoproterozoic phosphorites and a contribution to the identification of biosignatures in phosphatic sediments.

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Field Sites

Project Progress

1. Preservation of Molecular Biosignatures

Differences in carbon isotope ratios of co-occurring dolomite, carbonate fluorapatite, and organic matter in Paleoproterozoic stromatolitic phosphorites from the Aravalli Supergroup, India, suggest elevated primary productivity during sedimentation. Postdoctoral Fellow Dominic Papineau’s research provides a basis for the examination of other Paleoproterozoic phosphorites and a contribution to the identification of biosignatures in phosphatic sediments.

Co-Investigator Robert Hazen collaborated with Postdoctoral Student Emre Unal and William Zinsmeister of the Department of Earth and Atmospheric Sciences, Purdue University, and Co-Investigator Andrew Steele on a study of oncoids (algally and/or cyanobacterially coated, loose, concretionary stromatolite-related objects, typically the size of peas) from the Cambrian Chambliss Limestone and Cadiz Formation of the Marble Mountains, Mojave Desert, California (see Figure 1). These objects preserve traces of possibly biogenic carbon, so the group is employing electron microprobe and micro-Raman analyses to characterize the samples. Their preliminary work suggests widely dispersed kerogenous-like material, comprising less than 0.1% by weight of the sample. There is no clear morphological significance to the spatial distribution, so it’s not clear that any definitive conclusions can be drawn regarding the biology. Nevertheless, these distinctive objects deserve further study, as they are common and distinctive in formations up to 3 Ga in age.

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2. Stable Isotope Biosignatures

Low molecular weight gaseous hydrocarbons have been detected in the atmospheres and at the surfaces of planets and moons such as Mars, Titan, and Europa. These gases are of great interest to Postdoctoral Fellow Penny Morrill because they often indicate biological reactions, and thus can be signatures of life. However, gaseous hydrocarbons can also be formed by abiogenic chemical reactions (e.g., photochemistry, hydrothermal reactions) in the absence of life. Of these reactions, serpentinization is of particular interest. Through hydration of ultramafic rock this process produces hydrogen gas and the reducing conditions necessary for abiogenic hydrocarbon synthesis, but it also produces conditions amenable for chemolithotrophic life. Serpentinization is a suspected source of hydrocarbons detected on Mars, Titan, and Europa and may have played an important role in the prebiotic chemistry of early Earth.

Diagnostic tools for identifying hydrocarbons with biologically formed carbon-compound precursors (biogenic hydrocarbons) and hydrocarbons with inorganic carbon-compound precursors (abiogenic hydrocarbons) are required not only to determine the origin of gases on other planets but also to determine the origin of life on Earth. They are also required to determine the origin of reduced carbon in ancient rocks as old as 3.8 Ga, as well as to detect life in modern extreme environments. While geochemical indicators, such as stable carbon and hydrogen isotopic values, of biogenic hydrocarbons have been described, less is known about abiogenic hydrocarbons because little is known about the natural processes that produce them. Morrill has led both experimental studies to determine the geologic conditions required for abiogenic synthesis of gaseous hydrocarbons and field exploration at a terrestrial site of active serpentinization.

Laboratory experiments were designed to determine the geologically relevant conditions required for abiogenic synthesis of gaseous hydrocarbons and to determine unique geochemical indicators of these abiogenic hydrocarbons. At 250°C and 500 bars, carbon dioxide was reduced in the presence of a metal catalyst under hydrothermal conditions in sealed gold capsules. Both iron powder and awaurite were excellent catalysts. Hydrocarbon chains up to six carbons long were produced. It was found that the concentration of methane increased with catalytic surface area, suggesting that the reactions are surfaced catalyzed. These gases had relatively large carbon isotopic depletions that mimic those of gases with biogenic origins. Hydrogen isotopes were also measured. A hydrogen isotopic enrichment trend was observed with each additional carbon. Similar hydrogen isotopic enrichments were also seen in Canadian Shield hydrocarbons interpreted as abiogenic. These experimental data further emphasize the need for both carbon and hydrogen isotope values to be used as key parameters for distinguishing biogenic from abiogenic gases.

To investigate the possible abiogenic origin of hydrocarbons at a site of active serpentinization, compositional and isotopic analyses were performed on gases collected from ultrabasic reducing springs flowing through serpentine and peridotite rocks in northern California. The sampled gases were primarily composed of nitrogen (≤ 72%), hydrogen (≤ 50%), and methane (≤ 17%). Other minor gases included argon and higher molecular weight hydrocarbons. The carbon and hydrogen isotope values of methane, with mean values of —64 and -335 ‰, respectively, indicate a biological source, namely, microbial methanogenesis. Evidence for microbial anaerobic methane oxidation was also observed. A linear relationship with a negative slope was measured between the concentrations of carbon dioxide and methane. The carbon isotopic ratios for both methane and carbon dioxide also point to an ongoing process of methane oxidation, such that as the concentrations of methane decrease and as the concentration of CO2 increases there is a carbon isotopic enrichment of 13C in the reactant and product, consistent with microbial oxidation of methane. This hypothesis is further supported by a hydrogen isotopic enrichment between the bubbling methane and the dissolved methane.

The Kamchatka Peninsula is situated in the far northeastern portion of Russia and spans a region of approximately 400 by 1200 km. Like Yellowstone National Park in Wyoming, Kamchatka represents a modern analogue for hydrothermal ecosystems that have been active throughout much of Earth’s history. This region contains 30 active volcanoes together with 270 exposed hydrothermal sites. The most powerful hydrothermal systems are located within a narrow (30—50 km) zone along the axis of the main regional deep faults, which cross the eastern and central Kamchatka volcanic belts. Oil seeps have been known in the central part of the Uzon caldera since the 1960s and were first explored by Menjailov and colleagues. The colorless to yellow oils are present as water emulsions in porous rocks beneath the thin (5—8 cm) surface bed of clay, most often near hot mud pools and submerged griffons of thermal water. Oil samples were collected by Co-Investigator David Deamer and his team by skimming from surface films on hot water in holes (excavated to a 20—30 cm depth) dug near the central boiling pool of the Vostochnoye field in the central part of the Uzon hydrothermal system. The overall compositions of these oils indicate that they are derived from hydrothermal alteration of algal/bacterial mat detritus buried by the volcanic ashfall deposits of the Uzon caldera and they are an admixture of n-alkanes formed slowly over an extended time interval. 14C dating suggests that the biological source of the oils was buried approximately 1100 years ago. The aliphatic and biomarker compositions of these oils are similar to those reported for the Waiotapu oil samples. However, the Uzon oils differ in composition from those in lakes of tectonic rift basins such as Lake Tanganyika and Lake Chapala by their high concentrations of alkanes, isoprenoids, and steranes, indicating a different mode or sequence of formation.

X-ray absorption near edge spectroscopy (XANES) coupled with scanning transmission X-ray microscope (STXM) and micro Raman spectroscopy have been applied to the molecular characterization of kerogen (type II-S) from Neogene sediments of the Shinjo Basin, Yamagata, Japan. The sediments have been known to be oil source rocks deposited approximately from 3 to 8 Ma, and intense oil generation occurred in the lower part of the sequence. Postdoctoral Fellows Hikaru Yabuta and Henner Busemann have focused on these samples as a test case for establishing chemical modification trends from biological materials to complex geopolymeric materials. To date they have analyzed 14 kerogens extracted from sediment samples spanning the 1600-m-thick sedimentary sequence. This work relied on the instrumentation at Beamline 5.3.2., Advanced Light Source, Lawrence Berkeley National Laboratory (XANES with STXM) and the Carnegie Institution of Washington (Raman). The results are still preliminary, but they could indicate a modification of chemical structure of the kerogen with the progress of diagenesis (thermal maturation) and/or change of sources (terrestrial plant versus algae) along with sedimentary depth, which will provide interesting insights to oil generation mechanisms.